1. Field of the Invention
The present invention relates to a fuel cell formed by stacking an electrolyte electrode assembly and separators alternately in a stacking direction. The electrolyte electrode assembly includes a pair of electrodes and an electrolyte interposed between the electrodes. Reactant gas passages and coolant passages extend through the fuel cell in the stacking direction.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a polymer ion exchange membrane as a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly. Each of the anode and the cathode is made of electrode catalyst and porous carbon. The membrane electrode assembly is sandwiched between separators (bipolar plates) to form the fuel cell. In use, generally, a predetermined number of the fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas (reactant gas) such as a gas chiefly containing hydrogen (hereinafter also referred to as the hydrogen-containing gas) is supplied to the anode. An oxidizing gas (reactant gas) such as a gas chiefly containing oxygen (hereinafter also referred to as the oxygen-containing gas) is supplied to the cathode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
In the fuel cell, a fuel gas flow field is formed on the separator facing the anode for supplying the fuel gas to the anode. An oxygen-containing gas flow field is formed on the separator facing the cathode for supplying the oxygen-containing gas to the cathode. Further, a coolant flow field is provided between the anode side separator and the cathode side separator such that a coolant flows along the surfaces of the separators.
Normally, the separators of this type are formed of carbon material. However, it has been found that it is not possible to produce a thin separator using the carbon material due to factors such as the strength. Therefore, recently, attempts to reduce the overall size and weight of the fuel cell using a separator formed of a thin metal plate (hereinafter also referred as the metal separator) have been made. In comparison with the carbon separator, the metal separator has the higher strength, and it is possible to produce a thin metal separator easily. The desired reactant flow field can be formed on the metal separator by press forming to achieve reduction in the thickness of the metal separator, and reduce the overall size and weight of the fuel cell.
However, in the case the thin metal plate is formed into the metal separator having the reactant gas flow field fabricated by press forming, the reactant gas flow field and the coolant flow field are formed on both surfaces of the metal separator. That is, the shape of the coolant flow field is determined inevitably based on the shape of the reactant gas flow field. In particular, in order to achieve the long grooves, assuming that the reactant gas flow field comprises serpentine flow grooves extending along the electrode surface, the shape of the coolant flow field is significantly constrained. Therefore, the flow rate of the coolant in the electrode surface is not uniform.
For example, a solid polymer electrolyte fuel cell disclosed in Japanese Laid-Open Patent Publication 8-180883 is known. The fuel cell is directed to achieve the required cross sectional area in each of the reactant gas flow field and the coolant flow field. As shown in FIG. 12, the fuel cell 1 is sandwiched between separators 2. The fuel cell 1 includes an electrolyte membrane 3 and electrode 4a, 4b formed on both surfaces of the electrolyte membrane 3.
On the surfaces of the separators 2 which face each other, support bodies 6a, 6b forming reactant gas flow fields 5a, 5b between the electrode membranes 4a, 4b and the separators 2 are provided. On the surfaces of the separators 2 which are stacked together, support bodies 7 are provided. The support bodies 7 abut against each other to form a coolant flow field 8. The coolant flow field 8 is connected to bridges 8a provided near a coolant inlet and a coolant outlet. Buffers 9 are provided on opposite sides of the electrode membranes 4a, 4b. 
However, in the buffers 9, buffers of three kinds of fluid, i.e., an oxygen-containing gas, a fuel gas, and a coolant are overlapped with each other in the stacking direction. Therefore, when the thickness of the fuel cell is thin, the desired height of the flow field is not achieved in the buffers 9. Thus, in the buffers 9, the oxygen-containing gas, the fuel gas, and the coolant are not distributed smoothly in the respective flow grooves.